@muthu kumar Answer is not 30 bcoz, when you do $^4C_2$ and select say 1,2,3,4, then according to you there are 2 combinations,.. 1,2,3,4,5,6 and 1,2,3,4,6,5
Now if you select say 3,4,5,6 then also according to you there are 2 combinations.. 1,2,3,4,5,6 and 2,1,3,4,5,6
Look here 1,2,3,4,5,6 is repeating..